"Hole Saw" 3D Touchprobe

[simonrafferty]

YouTube - CNC 3D Touch Probe
I've been building an Automatic Tool Changer for my Lathe - which has stalled while I wait for a Gecko driver to arrive - so my lathe is out of action. I decided to build a 3D touch-probe for my mill - but needed to make it without using the lathe.

Although my mill is perfectly servicable - I thought it would be fun to make this with the absolute minimum of technology.

The result is a very simple 3D probe that you can make using only a drill press and a couple of hole-saws - hence the name!

This was based on the execlent work done by "BrusselSprout" and Mr Bean (on here).

To make this you'll need:

4" x 2" of copper clad board.
3 x 3/8" Ball Bearings - though the design would scale to whatever size you've got.
6" of M4 threaded rod and 13 plain nuts
2 x M6 Tapped Stand-off pillars. Easy to make if you have a lathe, but thes ones came out of an old computer UPS.
1 x weak compression spring. Mine is 1" diameter, 1" long with 8 coils of 1mm wire. It came from an assortment set I bought from Screwfix.com.

In the pictures, you can see how I've used a 2 1/4" hole saw to cut out two disks which are about 2" in diameter out of the copper clad board. I have then used a smaller 1" hole saw to remove the middle from one and to cut through the copper claddin on the other.

I have drilled three equally spaced 7 mm holes in which the ball bearings will sit in each.

In the one with the 1" hole in the middle, I have drilled three 4mm holes, then used a 10mm end mill to remove an island of the copper cladding around the hole to insulate the threaded rod from the copper.

In the other disk, I have drilled three 8mm holes coresponding to the 4mm holes in the other disk.

I also cut a similar disk out of 1/8" Aluminium sheet and drilled three 4mm holes as above.

I have insulated the M4 threaded rod using 6mm Pneumatic hose which slides over easily.

Once the holes were drilled in the two copper clad disks, I cut the lines in the copper as shown using a hack-saw blade (you could mill them easily enough though).

The Stand-off pillars were tapped M6 and had an outside diameter of 3/8" - so easy to hold in a collet chuck in the mill.

I used a pair of M6 x 10 hex screws, one to hold one pillar to the Aluminium disk and the other to attach the copper clad disk with the 1/4" hole in the middle to the other pillar.

My original touch probe tip was just a long M6 Stainless cap bolt - although I have since cheated and used the lathe to turn a ball on the end of the original bolt.

you can see that the cuts in the disks divide the copper into a big island that incorporates two ball bearings and a small island with one bearing.

The electrical connections go to the small islands. Just solder a thin wire on to each - thin enough that it does not stop the middle disk from moving.

Arrange the stack of disks and ball bearings so that one small island is directly on top of the other small island. Then rotate the top disk 120 degrees such that the small islands do not line up.

If all is correct, the current needs to flow from one small island, through all three ball bearings to the other small island. Thus any of the bearings loosing contact will break the circuit.

Use a continuity tester to make sure there is no contact between the three threaded rods and the lower copper clad disk. If there is, you will get spurious results when probing!

This took me about three hours from start to finish to make.

Si

[ecurb5]

This looks pretty cool. What's the accuracy or how sensitive is it?

[simonrafferty]

It's surprisingly good. The main problem is the copper is exposed and tends to oxidise and break contact - so it needs cleaning if it's been left a long time.

The repeatability of it is within a couple of thou. I used it this week to map the surface of a complex shape for which I needed to machine a tight fitting sleeve. I've 3D printed a prototype - and it looks pretty good. Next just need to mill it in duplex stainless!

It was the kind of shape you'd spend weeks getting right if you were trying to measure it with a vernier and draw it on the CAD - I guess that's what a 3D probe is meant for!

Si

[ecurb5]

Originally Posted by simonrafferty It's surprisingly good. The main problem is the copper is exposed and tends to oxidise and break contact - so it needs cleaning if it's been left a long time. ....

Have you thought about solder coating the contact area?